1. Field of the Invention
The present invention relates to well logging and more particularly to neutron activation well logging for the determination, in a direct manner, of oil sands from water sands.
2. Description of Prior Art
With the advent of pulsed neutron well logging, it has become common practice in well workovers to log cased well bores with some type of pulsed neutron log in order to determine the water saturation and hydrocarbon content of sands disposed externally of the casing in a cased well bore. While the neutron lifetime well log using a pulsed neutron source had proven to be an accurate and reliable indicator of sands containing salt waters in the pore spaces between the sand grains, it often occurs in various areas of the world in which petroleum is being produced that the hydrocarbon located in the pore spaces of the sands is associated with fresh water rather than with salt water. Since the neutron lifetime (or thermal neutron decay time) well log is heavily dependent upon the chlorine content of the fluid filling the pore spaces in the earth formation surrounding the borehole, the log is ambigious in cases in which such spaces are filled with a fresh water and oil mixture rather than a salt water and oil mixture. In these cases, it is desirable to employ some other type well log which can distinguish the fluids in the pore spaces of the earth formation wherein fresh water is contained in the pore spaces rather than salt water or hydrocarbons.
It has been proposed in the prior art to perform neutron activation well logs based on the neutron activation of the element oxygen which is contained in the earth formations surrounding the well bore hole. The prior art has also proposed performing neutron activation well logs based on the neutron activation of the element silicon which is also contained in earth formations surrounding a well bore hole. In these prior art proposals however, it has never been proposed to simultaneously make a determination of the oxygen and silicon content of earth formations surrounding the well bore using neutron activation well logging techniques. Typically, in the prior art proposals for neutron activation well logging, it has been proposed to irradiate earth formations surrounding the well bore hole with neutrons and to utilize and extremely far spaced detector (from the neutron source) to detect gamma rays due to the decay of the unstable radioactive isotope aluminum 28 which is produced by the neutron activation of silicon in earth formations surrounding a bore hole. Because aluminum 28 has a relatively long decay half life, 2.3 minutes the gamma ray detector to neutron source spacing proposed in prior art suggestions for this type of well logging as been extremely long.
Similarly, prior art proposals of oxygen activation logging of earth formations in the vicinity of a well bore hole generally have provided for means for irradiating the earth formation surrounding the bore hole with high energy (or fast) neutrons of sufficient energy to provide the transmutation of the oxygen 16 nucleus to the unstable radioactive nucleus nitrogen 16. By then monitoring the 6.13 mev gamma ray emitted by the decay of nitrogen 16 to a stable isotope the oxygen content of earth formations can be measured. The radioactive half life of this nuclear reaction is 7.3 seconds which is very short compared with the 2.3 minute radioactive half life associated with the decay of aluminum 28. For this reason it has generally been considered incompatible in the prior art to perform a simultaneous log of both the silicon and oxygen components of earth formation in the vicinity of the well bore hole.
Accordingly, it is an object of the present invention to provide method and apparatus for performing neutron activation well logging in a simultaneous manner of both the oxygen and silion components of earth formations surrounding a well bore hole.
It is another object of the present invention to provide a simultaneous oxygen and silicon neutron activation well log utilizing a single high energy neutron source and a single detector to perform the simultaneous well log.
A still further object of the present invention is to provide a simultaneous silicon and oxygen neutron activation well log which can be interpreted as a direct indicator of hydrocarbon in the earth formation surrounding a well bore hole.